It's the latest start in 24 years, Joseph Bresee, MD, chief of epidemiology and prevention at the CDC's flu branch, said at a news teleconference.

The CDC declares a flu season when for three weeks in a row at least 10% of people seeing a doctor for flu-like symptoms actually test positive for flu. The CDC has done this for 29 years. Only once before, in the 1987-88 flu season, has it come so late in the year.